Method for making thin-film semiconductors based on i-III-vi&lt;sb&gt;2&lt;/sb&gt; compounds, for photovoltaic applications

ABSTRACT

The invention concerns a method for making thin-film CIGS which consists in: electrochemically depositing on a substrate a layer of stoichiometry close to CuInSe 2 ; then rapidly annealing said layer from a light source with pulses of sufficient power to recrystallize CIS. Advantageously, the electrodeposited elements are premixed. Thus, after the deposition step, a homogeneous matrix is obtained which can support sudden temperature increases during the rapid annealing.

The invention concerns the field of semiconductor thin film depositionfor photovoltaic applications.

Thin films of copper and indium and/or gallium diselenide and/ordisulfide (CIGS) are deposited on a substrate in order to fabricatephotovoltaic cells. Such compounds of general formulaCuGa_(x)In_(1-x)Se_(2-y)S_(y) (with x between 0 and 1 and y between 0and 2) are regarded as highly promising and could constitute the nextgeneration of thin-film solar cells. CIGS semiconductor materials have adirect band gap of between 1.0 and 1.6 eV, which permits strongabsorption of solar radiation in the visible range. Record conversionefficiencies of more than 18% have recently been obtained with cells ofsmall surface areas.

CIGS are also referred to as I-III-VI₂, referring to the chemical natureof their constituents, where:

-   -   the element Cu represents an element from column I (column 1B of        the periodic table),    -   the element In and/or the element Ga represent elements from        column III (column 3B of the periodic table), and    -   the element Se and/or the element S represent an element from        column VI (column 6B of the periodic table).

There are therefore approximately two column VI atoms per column I atomand per column III atom in the monophase domain around the I-III-VI₂composition of the CIGS.

The CIGS layers used for photovoltaic conversion need to have a p-typesemiconductor character and good charge transport properties. Thesecharge transport properties are favored by good crystallinity. The CIGSthus need to be at least partially crystallized in order to havesufficient photovoltaic properties for their use in the fabrication ofsolar cells. Crystallized CIGS compounds have a crystallographicstructure corresponding to the chalcopyrite or sphalerite systems,depending on the deposition temperature.

When they are deposited at a low temperature (precursor deposition),CIGS thin films are weakly crystallized or amorphous, and annealing ofthe layers has to be carried out by supplying heat in order to obtain animprovement of the crystallization of the CIGS and satisfactory chargetransport properties.

At the temperatures which allow at least partial crystallization of theCIGS, however, one of the constituent elements of the CIGS (principallyselenium Se) is more volatile than the other elements. It is thereforedifficult to obtain crystallized CIGS with the intended composition(close to the I-III-VI₂ stoichiometry) without adding selenium for theannealing of the precursor layer.

Furthermore, in order to obtain a p-type semiconductor character(conduction by holes) the composition of the layers should have a slightdeviation from the I-III-VI₂ stoichiometry in favor of the VI element.

In the fabrication of CIGS thin films for photovoltaic applications,therefore, the prior art embodiments involve annealing the precursordeposits in the presence of a selenium excess in the vapor phase.

The best photovoltaic conversion efficiencies (more than 17%) have beenobtained from CIGS when preparing thin films by evaporation. Evaporationis an expensive technique which is difficult to use on the industrialscale, however, particularly because of nonuniformity problems with thethin-film deposits over large surface areas and a low efficiency ofusing the primary materials.

Cathodic sputtering is more suitable for large surface areas, but itrequires very expensive vacuum equipment and precursor targets. The term“precursors” means intermediate compounds whose physicochemicalproperties are distinct from those of CIS (or CIGS) and make themincapable of photovoltaic conversion. They are initially deposited in athin-film form, and this thin film is subsequently processed in order toform the intended CIGS deposit.

Electrochemical deposition offers an advantageous alternative. Thedifficulties which are encountered, however, relate to controlling thequality of the electrodeposited precursors (composition, morphology) andprocessing them with a view to providing adequate electronic propertiesfor the photovoltaic conversion. Several approaches have been proposedin order to overcome these difficulties, including the following:

-   -   separate or sequential electrodeposition of the Cu then In        precursors, followed by addition of Se (a step referred to as        “selenization”), as described in U.S. Pat. No. 4,581,108;    -   electrodeposition of (Cu, In) binaries in the presence of an Se        suspension, as described in U.S. Pat. No. 5,275,714.

This is because it is easier to apply a single precursor at the time.

More recent developments (U.S. Pat. No. 5,730,852, U.S. Pat. No.5,804,054 ) propose electrodeposition which is equivalent to depositinga layer of precursors with the composition Cu_(x)In_(y)Ga_(z)Se_(n)(with x, y and z between 0 and 2, and n between 0 and 3) by using apulsed current method. The electrodeposition is followed by a step ofevaporating the elements In, Ga and Se in order to increase their levelscompared with the electrolyzed layer.

As regards “pure” electrodeposition, that is to say electrodepositionwithout an evaporation step and with the I-III-VI₂ stoichiometry, thebest efficiencies are about 6 to 7% as indicated in the followingpublications:

-   -   GUILLEMOLES et al., Advanced Materials, 6 (1994) 379;    -   GUILLEMOLES et al., J. Appl. Phys., 79 (1996) 7293.

These publications furthermore indicate that better results are obtainedwhen the annealing is carried out under selenium vapor pressure, attemperatures in excess of 450° C., in a vacuum. Conventional annealingis then performed in a diffusion furnace, under elementary Se pressure.Such annealing, however, is relatively time-consuming (of the order ofone to a few hours).

Document U.S. Pat. No. 5,578,503 describes a two-step method whichfirstly involves deposition by cathodic sputtering then rapid annealing(with lamps) of the precursors deposited in this way. In particular Cu,In and Se precursor elements are deposited separately in an elementaryform (Cu(0), In(0) and Se(0)) or in the form of binaries (such asIn₂Se₃). The structure initially deposited before annealing is thusessentially heterogeneous and is in the form of a multiplicity ofsuccessive sheets (stacks of Cu⁰/In⁰/Se⁰ or Cu⁰/In₂Se₃, or a combinationof the two) in the thickness direction of the layer. This mixture ofprecursors subsequently undergoes rapid annealing, comprising atemperature rise followed by a holding time needed to homogenize the CISlayer. When it is deposited by sputtering and has a heterogeneousstructure in sheets, however, the thin film is not good at withstandingabrupt rises in temperature, especially in mechanical terms. Since thethermal expansion coefficient of the layer is spatially inhomogeneous,delamination problems occur with this layer during the annealing step.Although advantageous, such a procedure is therefore not yet completelysatisfactory.

More generally, the methods of deposition by evaporation or sputteringuse sources which commonly consist of pure elements, or sometimesbinaries but rarely ternaries. A difficulty arises when carrying outsuch methods. It involves transfer of the elements from the source tothe substrate. This transfer is not the same for all the elements, andthe evaporation speed or sputtering rate may differ from one element toanother. At a high temperature, in particular, the vapor pressures ofthe elements (their volatility) may differ significantly. This effect iscommensurately more problematic when there are a large number ofelements in the alloy to be obtained (ternary CIS, quaternary CIGS,etc.).

It is an object of the present invention to improve the situation.

To that end, it relates to a method of fabricating I-III-VI₂semiconductor alloys in thin films for photovoltaic applications,wherein:

-   a) a thin film of a precursor with a slightly excess composition of    VI element compared with a final desired alloy composition is    deposited on a substrate, the constituent elements of the precursor    being intimately mixed so as to give the precursor a structure    having nanometric alloy grains joined by phases which are richer in    VI element, and-   b) rapid annealing of the layer obtained in step a) is carried out    using an electromagnetic radiation source,    -   with an electromagnetic power greater than or of the order of a        few W/cm², which is sufficient to activate the VI element and        make all of said alloy grains react so as to improve the        crystallization of said layer, and    -   for durations less than or of the order of tens of seconds,        which are sufficiently short to obtain substantially said        desired composition in the layer and give said layer        photovoltaic properties.

The term “nanometric grains” means alloy grains, most of whichadvantageously have a physicochemical nature close to that of the alloyintended in step b) (in terms of both composition and bonds betweenatoms, in particular) and which may amount to several tens ofnanometers.

The grains then together form a matrix which is advantageously compactand capable of withstanding an abrupt temperature rise during theannealing step b).

Overall therefore, the present invention involves the combination of twosteps which consist in:

-   -   preparing a precursor in which the elements are intimately        bound, this precursor having a morphology capable of        withstanding the rapid annealing in step b), and    -   rapidly annealing this precursor, so that the kinetics of the        processing are fast enough to limit exodiffusion of the VI        element (owing to its volatility) during the annealing, while        allowing satisfactory crystallization of the layer.

Furthermore, as indicated above, the conservation of the VI element inthe layer after annealing imparts a p-type semiconductor characterfavorable for the photovoltaic conversion.

In a preferred embodiment, both for speed and for simplicity ofindustrial implementation, said thin film is deposited byelectrochemistry in step a).

In a preferred embodiment, the annealing in step b) is carried out byillumination, preferably direct illumination, using a light source. As avariant, the annealing may be carried out by induction.

For CIS (or CIGS) annealing in step b), the temperature of the layer ispreferably raised to more than 450° C.

Advantageously, the rapid annealing in step b) may be carried out bytransferring an illumination power density of the order of ten watts percm² to the thin film for durations less than or of the order of tenseconds.

The photovoltaic efficiency of the thin films obtained after annealingmay be of the order of or even more than 8% under these conditions.

Other characteristics and advantages of the present invention willbecome apparent on reading the following detailed description andstudying the appended drawings, in which:

FIG. 1 schematically represents a system for electrochemical depositionof a CIS thin film;

FIG. 2 schematically represents the appearance of the structure in theform of a precursor matrix before annealing, on the nanometric scale;

FIG. 3 represents a system for rapid annealing, by illuminating the thinfilm obtained by electrodeposition;

FIG. 4 schematically represents the thin-film structure of a cellintended for photovoltaic applications;

FIG. 5 illustrates, by way of example for CIS, pairings of durations(abscissa) and average illumination power densities (ordinate) making itpossible to at least partially crystallize a layer without degrading it;

FIG. 6 illustrates another representation (here with logarithmic scales)of the pairings of durations (abscissa) and energies delivered to thelayer (ordinate) making it possible to at least partially crystallize aCIS layer without degrading it, and to obtain the photovoltaicefficiencies indicated by way of illustration below the experimentalpoints;

FIG. 7 illustrates a profile as a function of time of a power densitytransferred to the layer during a light pulse, by way of example;

FIG. 8 illustrates X-ray intensity peaks obtained for variable θorientations in the analysis of a CIS layer which has beenelectrodeposited (solid curve) and processed by rapid annealing (brokencurve).

Referring to FIG. 1, thin films of copper and indium diselenide areobtained at room pressure and temperature by electrodeposition of aprecursor film CM on a glass substrate S previously covered withmolybdenum Mo (FIG. 4). Advantageously, the substrate S is initiallycovered with an additional electronic conductive layer, for examplemetallic or in an oxide form (not shown). This conductive layer mayfurthermore rest on one or more sublayers used for a specificapplication (diffusion barrier, mirror or the like) in the fabricationof photovoltaic cells.

Referring to FIG. 1, the electrodeposition is carried out in a bath Bcontaining an indium salt, a copper salt and dissolved selenium oxide.In order to obtain a CIGS thin film whose general compositioncorresponds substantially to CuGa_(x)In_(1-x)Se₂ (with x between 0 and1), it will be understood that the bath may furthermore contain agallium salt. In an even more sophisticated variant, sulfur salts (forexample sulfite or thiosulfate salts) are added to the solution so as toobtain a deposit which has a composition close toCuGa_(x)In_(1-x)Se_(2-y)S_(y) (with x between 0 and 1 and y between 0and 2). The salts are mixed during the deposition by using a rotaryagitator M immersed in the electrochemical tank B.

According to an advantageous characteristic, therefore, the thin film isobtained by electrodepositing a precursor whose constituent elements areintimately premixed.

The concentrations of the elements of the precursor (in the form ofsalts and oxide in the solution) are between 10⁻⁴ and 10⁻¹ mol/l. The pHof the solution is preferably fixed between 1 and 3. The potentialapplied to the molybdenum electrode (cathode Ca) is between −0.8 voltand −1.9 volts with respect to the reference electrode REF, here made ofmercury(I) sulfate.

Thin-film deposits with a thickness of between 0.1 and 3 μm are obtainedwith current densities of about 0.5 to 4 mA/cm².

As a nonlimiting example, a precursor deposit is formed from a bathwhose concentrations are as follows:

[Cu(SO₄)]=1.0·10⁻³ mol/l, [In₂(SO₄)₃]=6.0·10⁻³ mol/l, [H₂SeO₃]1.7·10⁻³mol/l, [Na₂(SO₄)]=0.1 mol/l.

The pH of the bath is 2 in this example. The precursors are deposited bycathodic reaction with a set potential, preferably −1 volt with respectto the mercury(I) sulfate reference electrode. The current density is −1mA/cm².

In so far as the copper and indium salts, as well as the dissolvedselenium oxide, are mixed in the solution of the bath B, a precursorwhose elements are intimately premixed is obtained at the end of theaforementioned electrodeposition step a).

The obtained precursor film is dense, adherent and has a homogeneousmorphology, and its composition is close to the stoichiometriccomposition Cu (25%), In (25%), Se (50%).

The following table indicates the atomic composition by percentage, atdifferent points (points 1 to 5) of a CIS precursor filmelectrodeposited on a substrate measuring about 10 cm², thesecompositions having been analyzed by an electron microprobe (WDX). Cu (%at.) In (% at.) Se (% at.) In/Cu Se(In + Cu) Point 1 22.5 24.3 53.31.080 1.14 Point 2 22.7 24.4 52.9 1.075 1.12 Point 3 22.6 24.6 52.81.088 1.12 Point 4 22.9 24.5 52.6 1.070 1.11 Point 5 22.9 24.3 52.81.061 1.12 Mean 22.72 24.42 52.88 1.075 1.122 Standard 0.179 0.130 0.2590.010 0.011 deviation

Referring to the column that relates to the selenium composition Se, aslight excess of selenium can nevertheless be observed compared with thequantity needed to combine with the copper in the form of Cu₂Se and withthe indium in the form of In₂Se₃. This selenium excess is about 10% inthe example described, although it may be even less than this value aswill be seen below.

The selenium excess is a favorable element for obtaining a p-typesemiconductor character of the layer after the annealing step, whichallows satisfactory photovoltaic conversion.

A slight indium excess of about 7% is likewise observed with respect tothe copper. This indium excess has also been found to play a favorablerole for obtaining the photovoltaic properties.

It is possible to verify that the deposit complies with a satisfactorycomposition uniformity in the plane of the thin film. The Applicantshave furthermore observed that this uniformity was also complied withthrough the layer thickness, in particular by carrying out fine analysessuch as SIMS (Secondary Ion Mass Spectroscopy) or EELS (Electron EnergyLoss Spectroscopy).

It is also possible to create a composition gradient in the depositedlayer, however, for example by varying the proportion of coppercomposition as a function of the thickness (in particular by controllingthe value of the electrochemical potential set during theelectrodeposition). Such a composition gradient advantageously makes itpossible to obtain confinement of the carriers in the intendedphotovoltaic applications.

Such fine analyses (SIMS, EELS) have furthermore shown that thecomposition of the electrodeposited precursor is mostly close to thatdesired for the final alloy, in grains GR measuring a few tens ofnanometers which are represented by way of illustration in FIG. 2. Inthe final composition of the alloy obtained after the rapid annealingstep, the selenium excess is much less than of the order of 1%, but issufficient to “dope” (as p-type doping at about 10¹⁶ cm⁻³) the annealedCIS. It will therefore be understood that the composition of the alloyobtained after annealing is very close to that of the I-III-VI₂stoichiometric alloy. It is substantially this composition which isessentially present in the grains GR of the electrodeposited precursorbefore annealing.

Referring to FIG. 2, the films obtained after the electrodeposition stepconsist of a matrix which is generally amorphous (or weakly crystallizedcompared with the alloy after annealing) but comprises mostly grains GRof CIS (crystallites of the order of tens of nanometers).

The term “matrix” means a composite nature of the layer which may have aplurality of possible phases: ternary (in the case of CIS), binary (forexample Cu_(x)Se with x close to 2, and In_(x)Sey with x close to 2 andy close to 3) or even elementary (selenium or the like). As indicatedabove, the grains GR have a composition close to that desired for thefinal alloy, for example CuInSe₂ in the case of CIS, while the precursoroverall has a selenium excess of about 10%. It is thus found that phasesPH richer in selenium are present outside the grains or at the surfaceof the grains as represented in FIG. 2 by way of example, for exampleelementary selenium Se or the binary CuSe.

The volume of the layer occupied by the grains GR is nevertheless verymuch predominant compared with that occupied by these phases PH. Theprecursor electrodeposited in this way overall has a physicochemicalnature mostly close to that of the final desired alloy, and not only interms of composition but also in terms of chemical bonds, which arethose of the desired alloy in the grains GR. The intimate mixing of theelements (and therefore the short migration distances of the elementsduring the annealing) thus contributes to the alloy not being degradedduring the annealing.

Further to the majority CIS grains, binary grains with compositionsclose to Cu₂Se and In₂Se₃ (not shown in FIG. 2) may also coexistlocally. During the rapid annealing step b), these grains are alsocapable of reacting together to give a coarser crystallized grain with acomposition close to CuInSe₂.

Even though the risks of exodiffusion during the rapid annealing arelimited, a layer that covers the precursor layer is furthermoredeposited before the annealing step in order to further limit anyexodiffusion of elements of the precursor, such as Se, during theannealing step.

According to an advantageous characteristic of the present invention, itis thus possible to further limit exodiffusion of the volatile elementsduring the annealing by applying a protective layer CP (FIG. 3) to thesurface of the CIS layer CM, before the annealing step. By way ofexample, a film of sodium chloride (or alternatively sodium or potassiumfluoride) may be used as a protective layer. It is formed by immersingthe precursor film in an aqueous solution of NaCl and drying in air.This protective layer is advantageously removed after the annealingtreatment, for example by dissolving (in an aqueous solution, forexample). In another embodiment, a surface layer of elementary seleniummay instead be provided in order to limit exodiffusion of the seleniumduring the annealing.

It will therefore be understood that the selenium excess in the initialprecursor may be reduced compared with the above table of compositions(to values of less than a few percent) when such a protective layer CPis added.

The good degree of oxidation of the elements mixed in this way duringthe electrodeposition step avoids uncontrolled chemical reactions duringthe annealing. In other words the free energy of reaction is relativelylow, which avoids the occurrence of highly exothermic reactions such asthose of the elementary indium and selenium in the CIS. In fact, theelements In and Se in the elementary form In⁰, Se⁰ are highly reactive.In particular for this reason, excessively rapid annealing in aso-called “sheet” deposition method as described above could lead todegradation of the layer.

This matrix is furthermore substantially homogeneous in the spatialdistribution of its expansion coefficient. It is therefore capable ofwithstanding an abrupt temperature rise during the step of annealing byillumination, compared with the aforementioned heterogeneous sheetstructure.

More generally, the physicochemical nature (degree of oxidation,density, thermal expansion coefficient) of the precursor is mostly veryclose to the CuInSe₂ crystallized phase, which greatly limits theoccurrence of inhomogeneities during the annealing (swelling, localizeddelamination). It will thus be understood that the matrix which isobtained after the electrodeposition step, and which comprises thepremixed elements of the precursor, advantageously makes it possible tocarry out rapid annealing, this matrix being capable of withstandingtransferred powers greater than or equal to 10 W/cm².

The precursor films have only weak photovoltaic properties per se afterthe deposition step. In fact, these photovoltaic properties are obtainedonly after a thermal annealing treatment. The recrystallization of thethin film makes it possible to obtain good charge transfer propertiesfor the photovoltaic conversion.

According to one of the characteristics of the invention, this heattreatment is carried out by rapidly annealing the electrodeposited thinfilm CM. Referring to FIG. 3, the thin film CM electrodeposited on thesubstrate S is arranged on a sample holder PE, which is preferablycapable of being moved in a horizontal plane (displacement along theaxis X as represented in FIG. 3) relative to a light source LA, which inthe example described is a bank of halogen lamps with a high radiationpower, advantageously in an optical absorption band of the thin film CM.In the example described, the term “rapid annealing” therefore meansillumination of the film CM so as to allow at least partialcrystallization of this thin film, for total durations of the order of10 seconds. This rapid annealing is carried out in a lamp furnace (FIG.3) in which the thin film can receive radiated powers of the order of 10W/cm² or more by direct incidence. As a variant, rapid annealing may becarried out by induction heating using a current loop.

The energy transferred to the thin film during the rapid annealingactivates the selenium (the selenium particularly being in excessoutside the grains GR), which initiates agglomeration of the grains GRas per sintering. The nanometric grains GR in the precursor jointogether to form coarser grains with a substantially micrometric size.During the rapid annealing, the excess selenium plays an important roleas an agent for recrystallization and defect passivation. This role isfavored in particular by the short interatomic distances that theselenium has to cover. During the annealing, the homogeneous matrixstructure of the precursor in turn plays an “internal lid” role for theselenium (by confinement) greatly limiting its exodiffusion.

The annealing may advantageously be carried out under atmosphericpressure, in ambient air, or alternatively under a neutral gas pressure(for example argon or nitrogen).

In the example described, the maximum power per unit area which the thinfilm actually receives is estimated at 25 W/cm², when considering therated power of the lamps, the dispersion of the light between the lampsand the thin film, the losses by reflection, and the like.

FIG. 7 represents a pulse with a maximum power timed for 3 seconds.Leading and trailing edges of the light power delivered as a function oftime can be seen, however, which are due to the inertia of the lamps.Now referring to FIG. 5, such a pulse has nevertheless made it possibleto anneal a CIS thin film, satisfactorily crystallizing it so as toobtain good photovoltaic properties.

FIG. 5 represents experimental points (dark squares) corresponding topairings of average light power/annealing duration which have made itpossible to obtain crystallized layers. The aforementioned 3-secondpulse corresponds to the first point on the left of the graphic. Thezones A, B and C delimited by the broken curves correspond respectivelyto:

-   -   power/duration pairings in which the power is too great (zone A)        and the layer is liable to be degraded during the annealing,    -   power/duration pairings making it possible to obtain        satisfactory crystallization of the layer (zone B), and    -   power/duration pairings in which the power is not sufficient for        annealing the layer correctly (zone C).

The working range of the annealing (zone B) in terms of “power/duration”pairing is thus delimited by a crystallization curve (lower power zone)and a film degradation curve (higher power zone). With high powers theApplicants sometimes observed degradation of the films, particularly theunderlying molybdenum film. With powers that are too low and/or timesthat are too long, the crystallization is insufficient and evaporationof the selenium simply risks taking place, the saturated vapor pressureof the selenium already being significant at 200° C.

An experimental point in FIG. 5 (other than the aforementioned firstpoint) may correspond to one or more successive light pulses, separatedby periods without illumination. A more precise representation of theseexperimental points is given in FIG. 6, showing “deliveredenergy/duration” pairings with logarithmic scales on the abscissa andordinate. With such scales, the ranges are delimited by substantiallylinear curves.

Of course, these ranges are represented by way of illustration in FIGS.5 and 6: they are merely intended to explain the phenomena involved.

The Applicants have nevertheless observed that with a layer thickness ofclose to one micron deposited on a glass substrate, the powertransferred to the layer must be more than a few watts per cm² in orderto commence sufficient crystallization. Advantageous annealing isobtained with a power in excess of 15 W/cm², for a duration less than afew tens of seconds.

It will be understood, however, that the selenium excess in the initialprecursor may be reduced if the annealing durations are shorter.

After annealing, the thin film CM is advantageously recrystallized in asubstantially equivalent or even better way compared with that which isobtained at the end of conventional annealing, under selenium vaporpressure, at temperatures in excess of 450° C. and for durations ofclose to one hour.

According to one of the advantages which the present invention offers,therefore, the premixed structure of the electrodeposited precursor iscapable of favoring the recrystallization process, but with durationsmuch shorter than those of conventional annealing.

In particular, the premixed structure obtained after theelectrodeposition step allows more rapid and more controllablecrystallization compared with the aforementioned method of deposition bysputtering thin films with the so-called “sheet” structure. This isbecause the electrolyzed matrix of precursors obtained after thedeposition step comprises constituents which are intimately mixed on theatomic scale, and these constituents have no need to diffuse over greatlengths (compared with the interatomic distances) in order to form theCuInSe₂ phase during the annealing.

In the conventional methods, furthermore, the quantity of excessselenium is customarily much more than 10% after the deposition step andbefore the annealing, which has a negative impact on the cost ofobtaining good photovoltaic materials. As indicated above, anothersolution of the prior art consists in selenization in the presence ofH₂Se gas. However, this gas is highly reactive and very toxic. With themethod to which the present invention relates, therefore, photovoltaicperformances are moreover obtained with relatively small initialquantities of selenium directly after the electrochemical deposition andwithout adding selenium during the annealing. Indeed, the rapid kineticsduring the annealing greatly limit exodiffusion of the element Se outfrom the layer. This effect advantageously makes it possible to obviatethe customary use which is made of the toxic gas H₂Se in theselenization methods, as well as any other addition of Se during theannealing step.

Nevertheless, the use of rapid annealing in the context of the presentinvention is also compatible with the addition of VI elements (forexample selenium or sulfur in order to increase the band gap of thesemiconductor) during the crystallization phase.

According to another advantage which the present invention offers, therelatively short annealing time makes it possible to limit the risks ofdeforming the substrate S as a result of very high temperatures. Thissubstrate may be made of glass but also of any other material which canwithstand minor temperature rises. This is because the combination ofdeposition from solution (by electrochemistry or by precipitation) atroom temperature and rapid annealing allows recrystallization of thefilms without the substrate having the time to be damaged in a way whichis detrimental to the intended application. Deposition on deformablesubstrates (for example made of plastic) may therefore be envisaged.Advantageously, a deposit on a polymer substrate such as KAPTON® may beannealed using one or more pulses with an illumination power of close to25 W/cm² and lasting a few seconds. Furthermore, satisfactory resultshave also been obtained with CIS electrodeposited on a metallicsubstrate, such as an aluminum sheet. The Applicants then observed thatthe metallic substrate can advantageously withstand the rapid annealingwhich allows satisfactory recrystallization of the CIS. Electrochemicaldeposition on a conducting metal substrate advantageously makes itpossible to obviate the underlying layer of molybdenum. Furthermore,deposition on a flexible substrate (such as an aluminum sheet or aKAPTON panel) advantageously makes it possible to produce materials forphotovoltaic applications which can be fitted on board spacecraft,because of their light weight and their capacity for folding.

FIG. 8 represents an X-ray diffraction diagram of a CIS film before andafter rapid annealing (respectively the solid curve and the brokencurve). Before annealing, the line in the (112) plane which ischaracteristic of CIS is very broad and not very intense, which showsthat the electrodeposited precursor is poorly crystallized. It is mostlythe ternary phase of the CIS which appears here, however, while thebinary phases or the elementary phases (for example Cu) appear virtuallynot at all. Essentially a superposition of the diffraction lines in the(112) plane of the CIS before and after annealing is thus observed.After annealing, however, the (112) line is finer and more intense.

After rapid annealing of the layer CM, photovoltaic cells are producedby depositing a thin layer of CdS and then a layer of ZnO by cathodicsputtering (FIG. 4). A final layer or an antireflection stack AR mayoptionally be provided so as to increase the efficiency of the solarcell. The cells produced in this way have photovoltaic propertiesleading to efficiencies of the order of 8% (FIG. 6) without gallium andwithout the antireflection layer AR represented in FIG. 4.

The combination of electrochemical deposition of premixed precursorelements and rapid annealing of the precursor layer thus leads to amethod which is more rapid, less expensive (since the salts present inthe bath B can be used in a subsequent electrolysis) and nonpolluting(thus eliminating the toxic release of H₂Se). Furthermore, thedeposition takes place at room temperature and pressure (respectivelyabout 20° C. and 1 atm). As indicated with reference to the table above,this method offers the possibility of processing large surface areaswith good uniformity.

Of course, the present invention is not limited to the embodimentdescribed above by way of example. It covers other variants.

A method for fabricating the CIS ternary was described above. Of course,deposition and processing by rapid annealing of the CIGS quaternary orthe CIGSeS penternary (with addition of sulfur) may be carried out undersubstantially identical conditions. In particular, the photovoltaicefficiency obtained with a CIGS layer would be substantially increasedcompared with that obtained from a CIS thin film.

It will be understood in particular that, being a VI element, the sulfurin a CuGa_(x)In_(1-x)Se_(2-y)S_(y) composition also plays the role of anagent for recrystallization and defect passivation of the thin filmduring the annealing. Likewise, an initial sulfur excess may lead to thep-type semiconductor character of the layer being obtained after theannealing.

Electrochemical precursor deposition making it possible to obtain amatrix with a physicochemical nature which is the same as that of thedesired alloy was described above. Other types of deposition fromsolution may nevertheless be provided, such as deposition byheterogeneous precipitation on a substrate.

More generally, yet other types of deposition may be envisaged. Byevaporation, a matrix of Cu—In—Ga—Se—S precursors is deposited with alow substrate temperature. The composition of the matrix is defined bythe evaporation rate of the pure elements in the respective sources. Ifthe substrate is not heated, then the matrix will not be verycrystallized but will have a composition which is easier to control.This matrix may furthermore undergo rapid annealing in the context ofthe present invention.

Furthermore, other semiconductor alloys of the chalcogenide type (alloysincluding an element from column VI of the periodic table (S, Se, Te)),such as cadmium telluride CdTe may also be used in order to fabricatephotovoltaic cells. The same problem of different respectivevolatilities between Cd and Te is observed when wishing to obtain a CdTelayer suitable for photovoltaic applications. However, CdTe thin filmscan be deposited and annealed according to the same steps of the methodin the context of the invention. For example, cadmium telluride (CdTe)films may be prepared by electrodeposition in a bath of cadmium sulfate(about 1 mol/l) saturated with dissolved tellurium oxide TeO₂, with anacid pH of around 2 or less. The deposition potential is −1 volt withrespect to a mercury(I) sulfate reference electrode. Rapid annealing inthe context of the present invention is then provided in order torecrystallize the layer.

1. A method of fabricating I-III-VI₂ semiconductor alloys in thin filmsfor photovoltaic applications, wherein: a) a thin film of a precursorwith a slightly excess composition of VI element compared with a finaldesired alloy composition is deposited on a substrate, the constituentelements of the precursor being intimately mixed so as to give theprecursor a structure having nanometric alloy grains joined by phaseswhich are richer in VI element, and b) rapid annealing of the layerobtained in step a) is carried out using an electromagnetic radiationsource, with an electromagnetic power greater than or of the order of afew W/cm², which is sufficient to activate the VI element and make allof said alloy grains react so as to improve the crystallization of saidlayer, and for durations less than or of the order of tens of seconds,which are sufficiently short to obtain substantially said desiredcomposition in the layer and give said layer photovoltaic properties. 2.The method as claimed in claim 1, wherein most of the alloy grainsobtained in step a) have a composition close to said desiredcomposition.
 3. The method as claimed in claim 1, wherein said desiredcomposition corresponds substantially to CuGa_(x)In_(1-x)Se_(2-y)S_(y)with x between 0 and 1 and y between 0 and
 2. 4. The method as claimedin claim 1, wherein the overall composition of the precursor has aslight excess of III element.
 5. The method as claimed in claim 1,wherein said thin film is deposited by electrochemistry in step a). 6.The method as claimed in claim 5, wherein an underlying layer ofmolybdenum is deposited on the substrate prior to step a).
 7. The methodas claimed in claim 5, wherein said desired composition correspondssubstantially to CuGa_(x)In_(1-x)S_(2-y) with x between 0 and 1 and ybetween 0 and 2 and wherein the electrodeposition is carried out in asubstantially acid bath comprising salts of copper and indium and/orgallium, as well as selenium oxide in dissolved form and/or sulfursalts.
 8. The method as claimed in claim 1, wherein said alloy grainshave a physicochemical nature close to that of the alloy obtained instep b), and together they form a matrix capable of withstanding anabrupt temperature rise during the annealing step b).
 9. The method asclaimed in claim 8, wherein the temperature of the layer is raised tomore than 450° C. in step b).
 10. The method as claimed in claim 9,wherein the power transferred to the layer is more than 5 W/cm².
 11. Themethod as claimed in claim 10, wherein the power transferred to thelayer is more than 10 W/cm², for a duration shorter than 30 seconds. 12.The method as claimed in claim 11, wherein the power transferred to thelayer is of the order of 20 W/cm², for a duration shorter than 10seconds.
 13. The method as claimed in claim 1, wherein the excess of VIelement in the overall composition of the precursor is less than or ofthe order of 10%.
 14. The method as claimed in claim 1, wherein thesubstrate is made of a plastic material.
 15. The method as claimed inclaim 1, wherein the substrate is made of a metallic material,preferably aluminum.
 16. The method as claimed in claim 1, wherein aprotective layer that covers the alloy layer, and which is capable oflimiting any exodiffusion of VI element out from the alloy layer duringthe annealing step b), is furthermore deposited in step a).
 17. Themethod as claimed in claim 16, wherein said protective layer is solubleat least after the annealing step b).